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How Bright Are the Stars?


Last updated 9/14/2023 at 10:50am

Ancient Greek astronomer Hipparchus created the original system of classifying a star's magnitude.

Much as we measure the intensity of natural phenomena such as earthquakes or tornadoes with a "magnitude," we also use the term to refer to a star's brightness. Unfortunately, many beginning stargazers are mystified by this concept.

It was the ancient Greek astronomer Hipparchus who, 21 centuries ago, compiled the first catalog of star brightnesses. In it, he listed the stars around the sky and rated their brightnesses on a scale of 1 to 6. He designated the brightest as first magnitude and the dimmest as sixth.

When astronomers began using modern technology to measure stellar brightnesses, they learned two interesting things. First, they found that each of Hipparchus' magnitudes is about 2.5 times brighter than the next greater magnitude. In other words, a second-magnitude star is about 2.5 times brighter than one of third magnitude. And, secondly, they found that there were objects brighter than first magnitude, as well as countless objects fainter than sixth. So, they expanded the scale to accommodate these.

For example, stars 2.5 times brighter than first magnitude were termed zero magnitude. Vega, the brightest star now appearing high in the east at dusk, is a good case in point. Stars brighter than this were assigned negative magnitudes, and those stars fainter than naked-eye visibility were assigned larger magnitudes.

Remember that number line you learned in school? Thought you'd never see it again, didn't you!

The range of celestial brightnesses is impressive. The sun shines at a stunning magnitude of -26, and the full moon appears at -12.5. This week, the planet Venus appearing low in the western sky at dusk shines at -4.7. The faintest objects seen by the largest professional telescopes have been measured to be about 30th magnitude.

Now, throwing around numbers like this can be interesting, but to see how this brightness scale works, go outdoors after dark and look midway up in the northern sky. There, Northern Hemisphere stargazers should spot Polaris, the North Star.

It's not the brightest in the heavens as many beginners think; it's rather faint – only a second-magnitude star.

Streaming upward from Polaris after dark this week are the stars of the Little Dipper. This grouping is tough to see from anywhere near the bright lights of a city. From there, you'd be lucky to find Polaris and maybe two additional stars, but drive to a dark site in the wilderness and you'll have much less trouble locating all seven stars of the Little Dipper.

Once you find it, you'll discover that each of the Little Dipper's "bowl" stars shines with a different brightness. The brightest is known as Kochab and appears much the same as Polaris: second magnitude. To the right of Kochab lies Pherkad, a third-magnitude star. Then comes Zeta Ursae Minoris – a fourth-magnitude star. And finally, we find Eta Ursae Minoris – a fifth-magnitude star.

Along with the stars of the Summer Triangle high in the eastern sky, you'll have a good set of comparison stars to estimate the magnitudes of others around the heavens. Once you begin to do this, you'll be able to estimate stellar magnitudes without using any of these comparison stars for help.

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